How can I merge objects while maintaining the materials in Sverchok?
We have not such node yet, and object in function. because was no needs.
Simple per polygon approach is here:
https://gist.github.com/81d278ada5a769c91d0a1fe9351dc53f
But in your particular case we can add index to shift materials, not universal solution:
https://gist.github.com/21a427f325312daa5086e635505430cf